The official U.S. time is kept on a cesium fountain clock named NIST-F1, located in Boulder, Colorado. On a recent trip to Boulder, Ira took a trip to see the clock. He spoke with Elizabeth Donley, acting head of the Time and Frequency Division at the National Institute of Standards and Technology, about keeping the official U.S. time on track—and how NIST is using advanced physics to develop ever more precise and stable ways to measure time.